Traction drive fluid for traction-type continuous variable transmissions (CVT) of automobiles must satisfy contradictory requirements that their traction coefficient is high even at high temperatures but their viscosity is low at low temperatures. Synthetic naphthenic compounds having a high traction coefficient at high temperatures are often problematic in that their flowability at low temperatures is poor. To solve this problem with them, some additive maybe added to them, which, however, will cause another problem in that the traction coefficient of the resulting mixtures may lower at higher temperatures. For example, Japanese Patent Laid-Open No. 2000-204386 discloses a traction drive fluid composition prepared by adding a poly-α-olefin to a naphthenic synthetic lubricant base oil. However, this is still problematic in that the additive, if added too much to the base oil, lowers the high-temperature traction coefficient of the resulting base oil over its additivity. On the other hand, it is generally desirable that the flash point of lube oil for automobiles is not lower than 150° C. in practical use.
The present invention has been made from the viewpoint mentioned above, and its object is to provide a lubricant base oil composition of which the flash point is not lower than 150° C. and which has good high-temperature traction characteristics and has improved low-temperature flowability characteristics.